GB2259573A

GB2259573A - Measuring and regulating the air concentration in a chamber

Info

Publication number: GB2259573A
Application number: GB9219289A
Authority: GB
Inventors: Didier Thibault
Original assignee: Superba SAS
Current assignee: Superba SAS
Priority date: 1991-09-11
Filing date: 1992-09-11
Publication date: 1993-03-17
Anticipated expiration: 2012-09-11
Also published as: DE4230343A1; FR2681137B1; FR2681137A1; GB9219289D0; GB2259573B

Abstract

Air concentration in chamber 1 is measured by solid electrolyte oxygen sensor 2 located in cavity 4 which is maintained at a substantially constant temperature equal to the maximum sensitivity temperature of the sensor 2. The sensor output voltage K1 is converted into air concentration (C1%), and is also compared with a reference voltage value (W), for controlling the air injection into or air removal from the chamber 1 when the voltage measured is less than or greater than the reference voltage value (W). A pressure sensor 9 may be used to obtain absolute values of oxygen concentration and the sensor temperature is preferably about 750 DEG C. The system may be used in installations for processing textile yarns by thermofixation, in an atmosphere containing air and steam. <IMAGE>

Description

"Device for measuring and regulating the air concentration in a chamber"

This invention relates to a device for measuring and regulating the air concentration in a chamber.

The invention relates, inter alia, to the application of this device to the regulation of the air concentration In a chamber containing a mixture of air and steam, such as a chamber for the continuous treatment of textile yarns by thermofixation.

In chambers or lines for the continuous processing of textile yarns by thermofixation, said yarns are processed by a mixture of air and steam at a given temperature and pressure.

The temperature and pressure are generally regulated these chambers and it is possible to control them perfectly.

However, applicants have found that the quality of the yarn obtained depends not only the processing pressure and temperature constant, but also on that of the ratio between the air and steam concentrations.

EP 0 421 672 discloses a device whereby it is possible to measure the air concentration of an oxygen-enriched medium from a signal delivered by a probe operating at low temperature. However, this probe does not allow exact measurement of the oxygen concentration when the latter is too low, since th.e output signal from the probe remains practically constant. Also, if the medium is humid, condensation phenomena may occur on the probe, with the risk of damaging it or rendering the measurements incorrect.

4 During thermofixation processing of textile yarns, the chamber contains an oxygen-depleied medium, which is very humid because of the water vapour present.

The object of this invention therefore is to provide a device whereby it is possible to measure and regulate the air concentration in a chamber continuously, more particularly in a chamber containing a humid medium of low oxygen concentration, such as that indicated above.

According to this invention this device is characterised in that it comprises:

an oxygen probe made of a material whose electrical conductivity varies in dependence on the oxygen concentration contained in the chamber, said probe being disposed in a cavity communicating with the chamber, - means for keeping the interior of the cavity at a substantially constant temperature equal to the maximum sensitivity temperature of the probe, means for measuring the voltage delivered by the probe, means for converting said voltage into air concentration, means for comparing said voltage with a reference voltage value, end means for controlling the air injection Into or the air removal from the chamber when the voltage measured is less than or greater than the reference voltage value.

The material of the oxygen probe has the property of absorbing a percentage of oxygen which is a function of the oxygen concentration in the chamber, and the effect of this is to modify its conductivity. This change in conductivity may be measured by the variation in the voltage measured at the probe output.

This voltage can be converted directly into oxygen and air concentration (given that the air contains a constant proportion of oxygen).

It is therefore possible to display and record this air concentration and, if the latter Is inadequate, ini!ct air into the chamber or, if the concentration is excessive, remove air therefrom.

The device according to the invention thus enables a substantially constant air proportion to be maintained in the chamber.

According to one advantageous version of the invention, it also comprises a pressure sensor measuring the pressure of the gas mixture in the chamber and delivering a voltage which varies in dependence on said pressure, means being provided to correct the value of the voltage measured by the oxygen probe, serving to determine the air concentration, in dependence on the pressure measured, in order to relate said air concentration to a predetermined reference pressure.

The device is therefore capable of keeping the proportion of oxygen constant even in the event of variation in the total pressure of the gas mixture in the chamber.

According to a preferred version of the invention, the material from which the oxygen probe is made is zirconium oxide.

This material has the property of having an electrical conductivity which varies very distinctly as a function of the oxygen concentration provided that it is brought to a temperature of about 750'C.

Other features and advantages of the invention will be apparent from the following description.

In the accompanying drawings, which are given by way of example without limiting force:

Fig. 1 is a diagrammatic general view of the device according to the invention.

1 Fig. a is a longitudinal section to an enlarged scale showing the device containing the oxygen probe disposed in a cavity communicating with a chamber.

Fig. 3 is a view showing the various functions of the regulator forming part of the device according to the invention.

Referring to the accompanying drawings, the device for measuring and regulating the air concentration in a chamber 1 comprises essentially:

an oxygen probe 2 consisting of a material 3 whose electrical conductivity varies in dependence on the oxygen concentration in the chamber 1, said probe 2 being disposed in a tubular cavity 4 communicating with the chamber 1 via an aperture 5, heating means 6 for keeping the inside of the cavity 4 at a substantially constant temperature equal to the maximum sensitivity temperature of the probe 2, means for measuring the voltage Ki delivered by the probe 2, - means for converting said voltage KI into air concentration, means for comparing said voltage Ki with reference voltage value W.

The above three means are contained in a regulator unit 7.

The device also comprises means for controlling the air injection into or the air removal from the chamber when the voltage measured is less then or greater than the reference voltage value W.

These means are contained in a control unit 8.

The device also comprises a pressure sensor 9 measuring the pressure of the gas mixture in the chamber 1 and delivering a voltage K2 which varies according to said pressure. Means contained in the unit 7 are provided to correct the voltage value K1 measured by the oxygen probe, serving for determination of the air concentration, in dependence on the pressure measured by the sensor 9 to relate said air concentration to a predetermined reference pressure.

Preferably, the material forming the oxygen probe 2 is zirconium oxide. The voltage delivered by the probe is then in accordance with the following equation:

) - X 110 C, (7 0,.)) where R is the gas constant, K.i.z P U, ( m p:

T is the probe operating temperature, n is 4, F is Faraday's constant, p is the absolute pressure in bar, C.(%02) is the oxygen concentration in the chamber and Cr=2) is the oxygen concentration of the reference ambient air.

Thus when the oxygen concentration in the chamber is low, the voltage delivered at the terminals of the zirconium probe increases very rapidly, even for small variations in the oxygen concentration.

With the probe, therefore, it is possible to obtain very accurate measurements of the oxygen concentration contained in the chamber.

In the example shown in Fig. 2, the cavity 4 containing the probe 2 has beating resistors 6 at its periphery, said resistors being connected to means for keeping the interior of the cavity 4 at a temperature of the order of 750C. The reason for this is that this is the temperature at which zirconium oxide has the greatest sensitivity to oxygen.

Also, at this temperature of 750C, measurements in a very humid medium rich in water vapour can be carried out without any risk of condensation on the probe, since condensation on the probe might damage the latter and render the measurements incorrect.

Also, the cavity 4 and most of the probe 2 are surrounded by a refractory covering.

Measurement and display of the temperature are effected in the unit 9 (see Fig. 2).

Unit 9 is connected to a temperature probe 10 extending near the zirconium oxide element 3.

Said unit 9 contains a component such as a thermostat which cuts off the electrical supply to the resistors 6 when the reference temperature (slightly above 7SO'C) is reached. Also, the thermostat re-starts the supply to the resistors 6 when the temperature falls below 7SO'C.

The various functions of the regulator unit 7 will now be described with reference to Fig. 3.

This unit is provided with two analog inputs El and E2. Input Ei receives the voltage Ki (in mV) delivered by the oxygen probe 2.

This voltage is then converted into air concentration Ci (%) in accordance with the formula:

Cl (%) = (100120.93) eXpt(67.03-RJ)/22.041 This formula has been determined from Nernst's law and the characteristics of the Zr02 oxygen probe used.

The input E2 receives the voltage K2 (in mV) delivered by the pressure sensor 9.

With this voltage K2 it is possible to correct the air concentration CiM in accordance with the following equation:

CM = CiM/K2 This air concentration is displayed on a screen 11 and is recorded on a printer connected to the output Si of unit 7.

The voltage corresponding to the air concentration C(%) is then compared, by means of a comparator 12, with a reference voltage W corresponding to the air concentration that it is required to maintain inside the chamber 1.

If the voltage is found to be less than the reference value W, the output S2 of the unit 7 applies to the control unit 8 a signal which controls the opening of a solenoid valve 8a which in turn controls the air injection to the interior of the chamber 1 until the voltage corresponding to the required air concentration is equal to the reference voltage W.

Also, if the voltage is found to be greater than W, OUtPUt S2 feeds a signal which controls partial evacuation of the gas in the chamber and injection of an air-depleted gas mixture.

Of course, if the air concentration in the chamber 1 i known, it is possible to determine its water vapour concentration, the total pressure of the mixture being measured by the sensor 9.

With the regulator used it is possible to obtain on/off or continuous regulation, with differential integral proportional control, which allows very sensitive response of the various regulating means (opening of solenoid valve Sa, partial gas evacuation, etc.).

The invention as described above is preferably applied to maintaining constant air and water vapour concentrations inside an installation for the thermofixation of textile yarns.

It has been found that the use of such a device in combination with an installation of the type described above enables the uniformity and quality of the processed yarns to be improved.

Of course the invention may also be applied to any other installation, textile or not, in which it is required to regulate the oxygen, air andlor water vapour content.

1 1 C L A 1 M S 1. A device for measuring and regulating the air concentration in a chamber (1), characterised in that it comprises:

an oxygen probe (2) made of a material whose electrical conductivity varies in dependence on the oxygen concentration contained in the chamber, said probe being disposed in a cavity (4) communicating with the chamber (1), means for keeping the interior of the cavity (4) at a substantially constant temperature equal to the maximum sensitivity temperature of the probe (2), means for measuring the voltage (Kt) delivered by the probe (2), means for converting said voltage (Kt) into air concentration (Ci%), means for comparing said voltage (Ki) with a reference voltage value (W), and means for controlling the air injection into or the air removal from the chamber (1) when the voltage measured is less than or greater than the reference voltage value (W).

2. A device according to claim 1, characterised in that it also comprises a pressure sensor (9) measuring the pressure of the gas mixture in the chamber (1) and delivering a voltage (K2) which varies in dependence on said pressure, means being provided to correct the value of the.voltage (K1) measured by the oxygen probe (2), serving to determine the air concentration, in dependence on the pressure measured, in order to refer said air concentration to a predetermined reference pressure.

3. A device according to claim 1 or 2, characterised in that the material from which the oxygen probe (2) i made is zirconium oxide.

4. A device according to claim 3, characterised in that the cavity (4) containing the probe (3) comprises heating resistors (6) connected to means (9) for maintaining the interior of the cavity (1) at a temperature of the order of 750'C.

5. Application of the device according to any one of claims 1 to 4 to the measurement and regulation of the air concentration in a chamber (1) containing a mixture of air and vapour.

6. Application according to claim 5, characterised in that the said chamber is a chamber for processing textile yarns by thermofixation.

7. Any novel feature or combination of features described herein.